The invention relates to a system for measuring thickness of material, and more specifically it relates primarily to a method and a sensor for measuring thickness distribution in a material by half-wave resonance in the material that is subjected to measurement. Furthermore, the invention relates to an automatic positioning system suitable for positioning one or more sensors in a system for measuring distribution of thickness in material in an object that is to be measured.
A number of materials used in pipelines, ship hulls, tanks and other structures are of such a nature that they may disintegrate or change with time as a consequence of corrosion, mechanical wear and so forth. This disintegration or change may result in the weakening of the supporting structural materials due to some of the material being discharged into the surrounding environment, or because materials alter their characteristic properties so that they no longer fulfil their desired aims. The effect of these processes of disintegration and change is often a reduction in the amount of the effective material. In structures of sheet material this may have the effect of reducing the sheet thickness. It is frequently found that the reduction of the material resulting from the aforementioned causes is not evenly distributed, but that at times there are large variations in thickness over relatively small areas, such as, for example, can be observed when a protective coating of paint on steel plate has been damaged in such a way that there is direct moisture contact with the steel, and where corrosion pitting has been allowed to develop. Similar corrosive attack can also be observed, e.g., on ship hulls. Since the actual hull is usually a part of the supporting structure of a ship, and since by being impenetrable to liquid it ensures both the ship""s buoyancy and that the ship""s load is not discharged into the surrounding environment, it is essential to be keep track of the state of the material at all times and to chart any disintegration of the structural material which may cause a weakening of the structure. Similar needs for monitoring and charting the state of material exist in a number of other areas in addition to that described above for ship hulls, such as bridge and building structures, pipes, boilers and tanks in industrial plants, road and rail vehicles.
Over the years, a number of different methods have been employed for measuring thickness of material, and among the non-destructive methods, measuring methods utilising sound waves have dominated in recent decades. In a common form, ultrasound is used for measurements according to the echo principle, where thickness of material is determined on the basis of time-of-flight measurements for a short ultrasonic signal, often at a single frequency, which is reflected from junctions between materials of different quality. A thickness measurement is thus made by registering the differences in time-of-flight between the ultrasonic signals returned by reflection, and from the registered differences in time-of-flight, thicknesses are calculated on the basis of knowledge of the speed of sound in the materials through which both the transmitted and reflected ultrasonic signals propagate. When measuring thickness using ultrasound according to the echo principle, the ultrasonic signal is transmitted from a measuring head in the form of a beam having a small cross-section and a relatively small angular aperture. The advantage of using a beam having a small cross-section and small angular aperture is that it reduces the probability of the measuring accuracy being impaired because the beam covers an area including material of varying thickness. However, this means that the area which is characterised is correspondingly small, and that the number of measurements necessary to cover a particular area thus increases correspondingly. In practice this means that such measurements on large structures are carried out at relatively long intervals because the total measuring period would otherwise be unacceptable. Consequently, when making such point-by-point measurements there will be large areas which in actual fact are not examined, and where it is conceivable that there might be unacceptable deviations from the desired thickness of material which will thus remain undiscovered by such a method.
Norwegian Patent Publication No. 179926 in the name of Red Band makes known a method for automatic status check, inspection, cleaning and/or surface treatment of structures, especially measurement of thickness of steel plate structures and pipes using ultrasonic signals from a remote-controlled, self-propelling unit. The self-propelling unit is moved continuously around the test area, and a transmitter transmits an ultrasonic signal in a direction substantially perpendicular to the surface of the structure. A reflected signal is received by a receiver, and thickness and material quality at the test point in question are determined on the basis of this signal, together with parameters such as time-of-flight for the reflected signal and material constants. The self-propelling unit effects its own positioning with the aid of known points in the structure. All data received about the wave form of the reflected signal is stored in a computer, and thickness and material quality are verified by comparing data for the received signal at one point with data for received signals at neighbouring points. The step is repeated to record data for new test points.
Another method for measuring thickness by means of sound waves is described, in for example, U.S. Pat. No. 3,844,166, wherein with the aid of registration of half-wave resonance in the object which is subjected to measurement it is possible to determine thickness of material in a rather limited area. The patent discloses a method and a device for measuring thickness, wherein ultrasound is used which is frequency-modulated in accordance with a sinusoidal law. The sound waves are emitted from a transducer which focuses the wave in towards a point at which the thickness of material is to be measured. Sound waves which are emitted from the object are received by a receiver means designed for the purpose. At frequencies where half-wave resonance occurs, a registration is made of the time it takes to count two predetermined numbers of whole periods of the acoustic signal, and the registered time then forms a part of a specified calculation formula whereby the thickness of material at the selected test point is determined. As for the previously described measuring method, this last-mentioned method also gives a single value for the thickness of material at one point as a result of each individual measurement.
Patent Application No. 153029, laid open by the Norwegian Patent Office, makes known an inspection apparatus for ultrasonic investigations of the wall of a pipe whilst the apparatus is moved along the interior surface of the pipe. The apparatus comprises at least one sensor in the form of a wheel probe wherein at least one ultrasonic transducer is located in a chamber filled with acoustic couplant, wherein the chamber is in the form of a rotatable wheel having a compact elastic tyre positioned adjoiningly over a rigid annular rim. The transducer is secured inside the wheel relative to the body of the inspection apparatus in a position where the emitted signal at all times is directed towards the point at which the wheel is in contact with the material that is being inspected, whilst the wheel turns as the apparatus moves along the surface of the material. The acoustic signal is coupled via the couplant, through the annular rim and then via the compact elastic tyre which is in contact with the surface of the material of the pipe that is to be inspected. The sensor is also provided with one or more elastic membranes which, for pressure equalising purposes, merely serve to take up any variations in volume which may occur in the couplant or in possible gas present in the wheel-shaped sensor chamber owing to temperature variations and partly owing to pressure variations when, for example, being raised or lowered. This measuring apparatus is thus limited, at each measuring sequence, to investigate only a very limited area of the material which is under the tangential points between the material surface and the wheel probe.
It is therefore desirable to have available a method and an apparatus which allow data to be obtained on each individual measurement over a large continuous area which characterises completely any varying thickness of material over the whole area. In cases where charting thicknesses of material comprises areas which have an extent greater than the area that can be covered by a measuring apparatus according to the invention, it is necessary to make a controlled number of measurements until the whole of the area in question has been covered, and at the same time determine the position of each individually measured area. Possibilities for controlling the measurements will also be of importance for measurements of known objects to be measured, as repetition of measurements during, e.g., periodic inspections, could be rendered considerably more efficient by concentrating the measuring activity in particular areas where earlier measurements have revealed substantial occurrences of changes in the material. Control of measurements on the basis of knowledge of the structure of the object to be measured will also be desirable in order to be able to make a more correct interpretation of the measuring results.
The present invention provides a new apparatus and a new method whereby it is possible to make an all-over measurement of the thickness distribution in the material of an object that is to be measured over a chosen continuous area of the said object.
Furthermore, the invention provides a new automatic positioning system for rapid and accurate position determination, which is suitable for determining the position of areas of an object which are subjected to thickness distribution measurements. A new measuring system is also provided which in addition to said new measuring apparatus and said new positioning system may also include one or more transport apparatuses and data transfer and processing systems, with which measuring system it is possible to make controlled measurements of thickness distribution in a material of a known object to be measured, and to collect, store and present the measuring results.